The present invention relates to an apparatus for compensating a wavefront aberration. More specifically, the invention relates to a wavefront aberration compensating apparatus for performing an aberration compensation which suppresses a wavefront aberration, as a factor for determining sharpness of an image when an object subjected to the aberration compensation, such as an eye for example, is observed, photographed and so forth at high magnification, to be small, and relates to an ophthalmologic unit having the same.
Conventionally, there is known a retinal camera which performs observation and photographing of a retina, by imaging the retina on the basis of a reflected light flux from the illuminated retina. However, since the reflected light flux from the retina passes through an ocular optical system including a cornea, a crystalline lens, a vitreous body for example, the retinal camera of this kind cannot obtain an image of the retina at high resolution, due to an influence of an aberration in the ocular optical system. Therefore, the conventional retinal camera has a problem in that a sharp image of the retina cannot be obtained, even attempting to perform observation, photographing and so forth of the retina at high magnification. Incidentally, the ocular optical system is far from an ideal optical element, possesses optical refractive properties which generate various aberrations such as myopia and astigmatism, and a wavefront due to the reflected light flux from the retina has distortions.
On the other hand, for example, Japanese patent application publication No. 2005-224328 proposes an apparatus capable of obtaining a sharp image of a retina even when a magnification is increased. The apparatus disclosed in JP2005-224328A is provided with an aberration measurement part which measures an optical aberration of an eye, and an aberration compensation part including a deformable mirror for compensating distortions of the wavefront of the reflected light flux caused by the optical aberration of the eye on the basis of a signal supplied from the aberration measurement part.
In a conventional technology, plural kinds of voltage variation templates such as a concentric template, a symmetrical template and an asymmetrical template are provided for adjusting a deformable mirror when a wavefront aberration is to be compensated by using the deformable mirror. The voltage variation templates are selected on the basis of the measured wavefront aberration, and one of voltage patterns as voltage values for respective electrodes is determined from the selected voltage variation template. The determination of the voltage patterns is repeated to perform the compensation of the wavefront aberration in which the deformable mirror is used.
Generally, an arithmetic processing method for the compensation of the wavefront aberration, in which the templates are utilized, includes a distortion, generated when a voltage is applied to a single electrode, as an influence function. The influence functions corresponding to the respective electrodes are superposed to calculate voltage alignment data corresponding to an objective configuration of the deformable mirror. Hence, since a unit of the templates is equivalent to the number of electrodes, an amount of calculation increases depending upon the number of electrodes.
Additionally, in the conventional technology in which three kinds of templates are provided, a congruence of the wavefront aberration to each expansion mode subjected to the compensation cannot be obtained by the templates of three kinds. Also, it is necessary to use the arithmetic processing method for the wavefront aberration compensation, in which the distortion generated when the voltage is applied to the predetermined electrode is included as the influence function, and to calculate a superposition coefficient used when the influence functions corresponding to the respective electrodes are superposed. The calculation for the superposition coefficients, however, requires time. Therefore, it is often said that the arithmetic processing method for the compensation of the wavefront aberration, in which the templates are used, is normally not suitable for the deformable mirror having the large number of electrodes.
Furthermore, the number of times of repetition of the compensation by the voltage patterns decreases when a target value for a residual aberration, as a difference between a wavefront aberration of an eye for example and an aberration compensated by the deformable mirror, is set to be large. Thus, the time required, for example, in photographing of a retina from initiation of the photographing to finishing of the photographing is shortened. However, a sharp image cannot be photographed when a high magnification is set.
In contrast, the sharp image is obtainable even when the high magnification is set, when the target value for the residual aberration is set to be small. However, the number of times for the repeated compensation by the voltage patterns is increased, and thus, the time required, for example, in the photographing of the retina from the initiation of the photographing to the finishing of the photographing becomes long.
Here, reasons why the control of compensating the deformable mirror is repeatedly performed by the voltage patterns, such that a shape of a thin-film mirror of the deformable mirror becomes nearer to the objective configuration, will be described.
The deformable mirror includes plural electrodes arranged at a predetermined interval on a back face of the thin-film mirror, and a voltage is applied to each of the electrodes, to deform the thin-film mirror only by a pulling force or an electrostatic force. In addition, since the thin-film mirror of the deformable mirror is a continuum, the respective electrodes cannot be treated individually for the shape deformation of the thin-film mirror. Hence, when one point of the thin-film mirror is pulled by one electrode, a part of the thin-film mirror corresponding to that one electrode is deformed largely, and at the same time, a part of the thin-film mirror corresponding to other electrodes is also deformed. Therefore, the compensation control of the deformable mirror is performed repeatedly by the voltage patterns, since the entire surface of the mirror is influenced when one part of the thin-film mirror is pulled.
Secondly, when a retina of an eye is to be photographed for example, a duration time in which a person can keep its eye open with good condition is several seconds for a person of shorter duration time, although such a duration time varies depending upon individuals. Thus, in order to complete a procedure from the initiation of the compensation of the wavefront aberration to the photographing within seconds, it is important that an optical system reach the aimed wavefront aberration with the minimum possible number of times of the compensation.
Additionally, in a field of photographing a retina of an eye for example, there has been a demand for photographing a sharp image at a high-magnification, such that a confirmation is possible to the extent of a visual cell of the retina, in order to increase accuracy in examination. To meet this demand, since the magnification can be made higher while sharpness of the image is maintained as the residual aberration becomes smaller, it is necessary to improve a limit of the aberration compensation by using the deformable mirror in which the number of electrodes, to which a voltage is applied, is large.
Therefore, in the compensation control of the deformable mirror, how to anticipate the voltage patterns for generating a configuration to compensate the wavefront aberration remains as a problem which still cannot be solved, in order for the optical system to reach the aimed wavefront aberration with the minimum possible number of times of the compensation from data on a shape of the aberration measured by a wavefront sensor, even when the deformable mirror having the large number of electrodes is used.